1. Field of the Invention
The present invention relates to a radio transmission apparatus and method, a radio reception apparatus and method, a transmitting and receiving method, and a recording medium for use in a mobile communication system according to a transmission diversity method based on Orthogonal Frequency Division Multiplexing-Code Division Multiplexing (OFDM-CDM).
2. Description of the Related Art
The more mobile communication systems are being developed, the higher the levels of the bandwidths, frequencies, and reliabilities thereof are needed. Thus, a transmission diversity method that improves transmission quality without extending a radio unit of a terminal becomes very desirable. Also, an OFDM-CDM method has been known as being very efficient in transmitting low-rate data or control data, or transmitting data in an environment in which an interference between cells are serious. Therefore, conventional transmission diversity-based methods will first be described, and then, a conventional OFDM-CDM-based method will be described.
Two examples of the transmission diversity-based methods will be described. One of the conventional transmission diversity-based methods uses two antenna branches. FIG. 1 is a block diagram of a radio transmitter 1d that uses 2-row, 2-column Space-Time Block Code (STBC) and has two transmission antennae 112-1 and 112-2, and a radio receiver 2d that receives signals from the radio transmitter 1d. The radio transmitter 1d space-time codes a pair of transmission symbols s=[s1, s2]T by using a 2-row, 2-column STBC unit 100, based on a 2-row, 2-column STBC matrix expressed in the following equation, and transmits the time-space coded symbols via the transmission antenna 112-1 and 112-2.
                    S        =                  [                                                                      s                  1                                                                              s                  2                                                                                                      -                                      s                    2                    *                                                                                                s                  1                  *                                                              ]                                    (        1        )            
The 2-row, 2-column 2-row, 2-column STBC unit 100 respectively transmits the symbol signals s1 and s2 via the transmission antennae 112-1 and 112-2 at a first instant of time. Then, the 2-row, 2-column STBC unit 100 respectively transmits the symbol signals −s2* and s1* via the transmission antennae 112-1 and 112-2 at a second instant of time. If the radio receiver 2d respectively receives signals r1 and r2 at the first instant of time and the second instant of time via a receiving antenna 120, the received signals r1 and r2 are expressed as follows:
                    {                                                                              r                  1                                =                                                                            h                      1                                        ⁢                                          s                      1                                                        +                                                            h                      2                                        ⁢                                          s                      2                                                                                                                                                                r                  2                                =                                                                            -                                              h                        1                                                              ⁢                                          s                      2                      *                                                        +                                                            h                      2                                        ⁢                                          s                      1                      *                                                                                                                              (        2        )            wherein h1 and h2 denote channel responses received from the radio receiver 2d via the transmission antennae 112-1 and 112-2. That is, h1 and h2 indicate values generated from the received signals by a channel estimation unit 102. A received signal vector is expressed in a matrix by using the values h1 and h2, as follows:
                                                        r              =                              [                                                                                                    r                        1                                                                                                                                                r                        2                        *                                                                                            ]                                                                                        =                                                [                                                                                                              h                          1                                                                                                                      h                          2                                                                                                                                                              h                          2                          *                                                                                                                      -                                                      h                            1                            *                                                                                                                                ]                                ⁢                                                                  [                                                                                                    s                        1                                                                                                                                                s                        2                                                                                            ]                                                                        (        3        )            
Also, a channel matrix H is expressed by using the values h1 and h2, as follows:
                    H        =                  [                                                                      h                  1                                                                              h                  2                                                                                                      h                  2                  *                                                                              -                                      h                    1                    *                                                                                ]                                    (        4        )            
A transmission signal estimated value vector ŝ is decoded by a time-space decoder 101 of the radio receiver 2d by using a conjugate transpose matrix HH of the channel matrix H, as follows:
                                                                        s                ^                            =                                                H                  H                                ⁢                r                                                                                        =                                                [                                                                                                                                                                                                                                      h                                1                                                                                                                    2                                                    +                                                                                                                                                  h                                2                                                                                                                    2                                                                                                                      0                                                                                                            0                                                                                                                                                                                                                      h                                1                                                                                                                    2                                                    +                                                                                                                                                  h                                2                                                                                                                    2                                                                                                                                ]                                ⁢                                                                  [                                                                                                    s                        1                                                                                                                                                s                        2                                                                                            ]                                                                                        =                                                (                                                                                                                                      h                          1                                                                                            2                                        +                                                                                                                    h                          2                                                                                            2                                                        )                                ⁢                s                                                                        (        5        )            
As expressed in Equation (5), since two symbol signals are transmitted in two time slots, the rate of transmission is a full rate, thus realizing a maximum rate of diversity of a combination of two transmission antenna branches.
Next, a closed loop transmission antenna diversity method, which is the other conventional transmission diversity-based method, which uses two pairs of 2-row, 2-column STBC and four antenna branches, will be described. FIG. 2 is a block diagram of a radio transmitter 1e that has four transmission antennae 112-1 through 112-4 and performs closed loop transmission antenna diversity, and a radio receiver 2e (see J. Akhtar and D. Gesbert, “Partial Feedback-Based Orthogonal Block Coding”, Proc. VTC′ 03-Spring, pp 287-291, Korea, April 2003).
2-row, 4-column STBC performed by a 2-row, 4-column time-space decoder 110 of the radio transmitter 1e is given by:
                    S        =                  [                                                                      s                  1                                                                              s                  2                                                                              b                  ⁢                                                                          ⁢                                      s                    1                                                                                                b                  ⁢                                                                          ⁢                                      s                    2                                                                                                                        -                                      s                    2                    *                                                                                                s                  1                  *                                                                                                  -                    b                                    ⁢                                                                          ⁢                                      s                    2                    *                                                                                                b                  ⁢                                                                          ⁢                                      s                    1                    *                                                                                ]                                    (        6        )            wherein b denotes binary code having a value of +1 or −1, computed by a feedback value calculator 126 of the radio receiver 2e. A method that uses the binary code b (hereinafter referred to as “control information”) is referred to as an STBC-Group Coherent Coding (STBC-GCC) method.
A received signal vector in the radio receiver 2e is expressed in a matrix, as follows:
                                                        r              =                              [                                                                                                    r                        1                                                                                                                                                r                        2                        *                                                                                            ]                                                                                        =                                                [                                                                                                                                          h                            1                                                    +                                                      b                            ⁢                                                                                                                  ⁢                                                          h                              3                                                                                                                                                                                                        h                            2                                                    +                                                      b                            ⁢                                                                                                                  ⁢                                                          h                              4                                                                                                                                                                                                                                                h                            2                            *                                                    +                                                      b                            ⁢                                                                                                                  ⁢                                                          h                              4                              *                                                                                                                                                                                                        -                                                          h                              1                              *                                                                                -                                                      b                            ⁢                                                                                                                  ⁢                                                          h                              3                              *                                                                                                                                                            ]                                ⁢                                                                  [                                                                                                    s                        1                                                                                                                                                s                        2                                                                                            ]                                                                        (        7        )            
Therefore, an equivalence channel matrix {tilde over (H)} is given by:
                              H          ~                =                  [                                                                                          h                    1                                    +                                      b                    ⁢                                                                                  ⁢                                          h                      3                                                                                                                                        h                    2                                    +                                      b                    ⁢                                                                                  ⁢                                          h                      4                                                                                                                                                                h                    2                    *                                    +                                      b                    ⁢                                                                                  ⁢                                          h                      4                      *                                                                                                                                        -                                          h                      1                      *                                                        -                                      b                    ⁢                                                                                  ⁢                                          h                      3                      *                                                                                                    ]                                    (        8        )            
In this case, a transmission signal estimated vector s may be decoded by using a conjugate transpose matrix {tilde over (H)}H of the equivalence channel matrix {tilde over (H)}, as follows:
                                                                        s                ^                            =                                                                    H                    ~                                    H                                ⁢                r                                                                                        =                                                [                                                                                                              α                          +                          β                                                                                            0                                                                                                            0                                                                                              α                          +                          β                                                                                                      ]                                ⁢                                                                  [                                                                                                    s                        1                                                                                                                                                s                        2                                                                                            ]                                                                                        =                                                (                                      α                    +                    β                                    )                                ⁢                                                                  ⁢                s                                                                        (        9        )            wherein α and β are respectively given by:
                    α        =                              ∑                          i              =              1                        4                    ⁢                                                                  h                i                                                    2                                              (        10        )            β=2b[Re(h1*h3)+Re(h2h4)]  (11)
In the STBC-GCC method, since a transmitting side controls the binary code b so that β has a positive value, two symbol signals can be transmitted in two time slots, and thus, the rate of transmission becomes a full rate. Also, both a space diversity gain a for four transmission antenna branches, and an array gain β are obtained.
Next, an example of the OFDM-CDM-based prior art will be described. FIG. 3 is a block diagram of a OFDM-CDM-based radio transmitter 1f and radio receiver 2f. FIG. 4 is a conceptual diagram of two-dimensional (2D) spreading applied to an OFDM-CDM method. Referring to FIG. 4, the 2D spreading is performed such that the original symbol signal is spread in the directions of frequency and time. The spreading of the original symbol signal in the direction of frequency is performed such that transmitted symbol signals are serial-to-parallel transformed by a serial-to-parallel transformer 31 of the radio transmitter 1f and the original symbol signal is copied as symbol signals corresponding to a rate of frequency-direction spreading SFFreq by copying units 132-1 through 132-n. The spreading of the original symbol signal in the direction of time is performed such that symbol signals, which are respectively copied by the copying units 132-1 through 132-n, are respectively spread by spreaders 1321-1, . . . , 1321-m, . . . , 132n-1, . . . , 132n-m based different spreading codes corresponding to a rate of time-direction spreading SFTime. In this case, in 2D spreading, since the rate of spreading SF=SFTime×SFFreq and each symbol signal is copied in the direction of frequency, the length of each symbol signal is repeatedly spread by using the spreading code corresponding to the rate of spreading SF at each rate of time-direction spreading SFTime. Also, the 2D spread signals are respectively allocated to sub carriers in a frequency domain, and input as sub carriers to a code multiplexer 133.
Next, a code multiplexer 133 code-multiplexes the spread signals that are spread by using different spreading codes. A transforming transmission unit 134 performs Inverse Fast Fourier Transform (hereinafter referred to as “IFFT”) on the code-multiplexed signals to transform the sub carriers into time-domain signals, inserts a Guard Interval (GI) into the time-domain signals to prevent an interference between code, and transmits the time-domain signals via a transmission antenna 135.
In the radio receiver 2f, a receiving transformer 142 removes the GI from each of the time-domain signals received via an receiving antenna 141, and performs Fast Fourier Transform (hereinafter referred to as “FFT”) on the time-domain signals to transform them into sub carriers. Next, the sub carriers are despread by despreaders 1431-1, . . . , 1431-m, . . . , 143n-1, . . . , 143n-m in the direction of time, and then, the despread signals are mixed by mixers 143-1 through 143-n in the direction of frequency. Lastly, the mixed signals are parallel-to-serial transformed by a parallel-to-serial transformer 144 in the direction of frequency, and then output as signals to be reproduced.
In an environment in which the above methods are actually performed, a receiving side includes a plurality of receivers. The above closed loop transmission antenna diversity method (the closed loop STBC-GCC) that uses four transmission antenna branches needs a feedback of control information from the receiving side, and therefore, receives control information from the plurality of the receivers. FIG. 5 is a block diagram of a radio transmitter 1g, and two radio receivers 2g-1 and 2g-2 that respectively provide feedbacks of control information b1 and b2.
However, when the same signal is transmitted to a plurality of users in an actual mobile telecommunication network, the radio transmitter 1g corresponds to a base station or an access point, and the radio receivers 2g-1 and 2g-2 correspond to mobile phones of the users. In this case, since a channel gain between the radio transmitter 1g and the radio receiver 2g-1 is different from a channel gain between the radio transmitter 1g and the radio receiver 2g-2, the feedbacks received from the radio receivers 2g-1 and 2g-2 are not always the same. Specifically, it is assumed that channel responses from four transmission antennae 112-1 through 112-4 of the radio transmitter 1g to the radio receiver 2g-1 are h1, h2, h3, and h4, and channel responses from the four transmission antennae 112-1 through 112-4 of the radio transmitter 1g to the radio receiver 2g-2 are h5, h6, h7, and h8. In this case, an array gain β1 in the radio receiver 2g-1 and an array gain β2 in the radio receiver 2g-2 are given by:β1=2b1{Re(h1*h3)+Re(h2h4)}  (12)β2=2b2{Re(h5*h7)+Re(h6h8)}  (13)
If the control information b1 has a value of 1 when the array gain β1 has a positive value and the control information b2 has a value of −1 when the array gain β2 has a positive value, the control information b1 and b2 have opposite values, thus preventing the radio transmitter 1g from being controlled. If both the control information b1 and b2 have the value of 1, the array gain β2 in the radio receiver 2g-2 has a negative value and the sum of gains α2+β2 in the radio receiver 2g-2 is less than α2, thereby causing degradation of transmission quality.